Helicopter rotors can be grouped into two general categories, hinged and hingeless. Hinged, or articulated, rotors permit movement both perpendicular and parallel to the plane of rotation of the blade to reduce blade stresses. Hingeless rotors eliminate the need for the hinges and utilize elastic deflections of the blade to relieve stress. Hingeless rotors provide reduced rotor complexity and cost as well as improved flying qualities and maneuverability.
Previously known hingeless rotor arrangements characteristicly suffer from a number of substantial drawbacks. For example, nearly all hingeless configurations are prone to aeroelastic instabilities involving rotor blade motions parallel to the plane of rotation. These instabilities can be characterized as isolated rotor instabilities not involving participation of the helicopter fuselage, or as coupled rotor-fuselage instabilities involving coupling between rotor blade motion and fuselage motion. In both cases, the elimination of blade hinges introduces strong structural coupling between individual blade motions and the combined rotor-fuselage motion. In many cases these coupling phenomena are undesirable and produce rotor instability. A particularly important class of instabilities involves a coupled rotor-fuselage instability called ground resonance. This instability may occur whenever the fundamental lead-lag bending frequency of the blade is less than the rotor rotational frequency. Instability problems such as those referred to are usually handled with trial and error design techniques and, in many instances, auxilliary lead-lag dampers must be installed to eliminate the instability. However, this solution, because of the cost and complexity thereof, tends to compromise the basic advantages provided by the use of a hingeless rotor.